4. Peroxide of nitrogen (NO.), was long supposed to possess acid properties, and hence was called hyponitric acid and formerly nitrous acid. It may be obtained in the gaseous, liquid, or solid form.
Gaseous peroxide of nitrogen is formed on mixing binoxide of nitrogen with half its volume of oxygen in an exhausted receiver ; immediate combination and condensation occur ; heat is evolved ; a red gas is formed, which is the peroxide of nitrogen, and when this coloured gas is subjected to a temperature of 0°, it becomes liquid, and sometimes solid. When also nitrate of lead is heated in a retort, it is decomposed, and red vapour of peroxide of nitrogen is emitted, which is condensed to the liquid state by being subjected to a freezing mixture. Towards the latter part of the operation the compound comes over in the anhydrous state, and may be obtained in crystals in a fresh, well-dried and cooled, receiver. The slightest trace of moisture prevents the formation of the crystals. Liquid peroxide of nitrogen is of a pale colour, but becomes darker when heated ; it emits a very strong disagreeable odour, and at a very moderate rise of temperature reassumes the gaseous form. It is decomposed by water with strong effervescence, being immediately separated into binoxide of nitrogen, which is evolved, and nitric acid, which remains in solution.
Nitric acid (NO.). Azotic acid.—This important acid may be formed by the direct combination of oxygen with nitrogen, or with any of the other lower oxides of nitrogen ; by decomposition of ammo nia in presence of oxygen ; by the passage of electric sparks (for example, lightning) through atmospheric air, or by the EREMAUCAUSIS of organic bodies that contain nitrogen. In combination with bases nitric acid is met with in nature in great abundance : it is rare to meet with a specimen of rain-water that does not contain traces of nitrate of ammonia ; most varieties of chalk contain a small quantity of nitrate of lime, and in many tropical climates, especially some parts of India and Peru, nitrates of potash and soda effloresce from the ground during the dry seasons of the year. The production of these nitrates is no doubt due to one, or a combination, of the causes already alluded to so far as the nitric is concerned, while the several bases arc always at hand in the soil in the state of carbonates or other salts decom posable by nitric acid. Native nitrate of potash constitutes the crude nitre, or saltpetre of commerce, and is the substance whence we derive nitric acid itself, and most of the other nitrates.
Ordinary liquid nitric acid was known so early as the 7th century ; Raymond Lully obtained it from nitre ; Geber termed it solutire water, and Glauber manufactured it by the agency of sulphuric acid, in precisely the manner adopted at the present day. Cavendish, however, first made known its composition in 1785 [CAVENDISH, HENRY, in Broo. Div.], and our knowledge of it has since been perfected by Davy, Gay-Lussac, Thompson, and others. On the small scale, nitric acid is conveniently obtained by heating in _a glass retort with a tolerably long neck, a mixture of equal weights of nitrate of potash and oil of vitriol ; inonohydrated nitric acid then distils over, and is readily condensed, while bisulphate of potash remains in the retort. The following equation represents the decomposition :— name of aqua fortis. It also becomes of the same colour when any organic matter, such as a piece of straw or paper is allowed to fall into it; even mere exposure to sunlight will produce the same decom position. The chief cause of the instability of nitric acid is the readiness with which it parts with a portion of oxygen ; indeed, as au oxidising agent, it is in constant request by the chemist. Metals thrown into it are rapidly converted into oxides with evolution of binoxide of nitrogen and production of orange-red nitrous fumes, a reaction taken advantage of in testing for the presence of nitric acid ; sometimes the binoxide of nitrogen is made to pass into solution of protosulphate of iron, in which it dissolves with formation of a black compound, a reaction that is even more delicate than that with the copper turnings ; it is performed as follows. Into the liquid suspected to contain nitric acid, or a nitrate, a few crystals of protosulphate of iron are placed, and the whole is agitated for a minute or two to facilitate the solution of a little of the salt ; strong sulphuric acid is now cautiously poured down the sides of the vessel, and being heavy flows on to the crystals at bottom, and at once gives rise to the black compound in question if a trace of nitric acid be present. The cause of the production of the compound is the oxidation of a part of the protosalt of iron to a persalt by the nitric acid, binoxide of nitrogen being evolved, and combining with the undecomposed protosulphate; the use of the sulphuric acid is to liberate nitric acid from combina tion, and to facilitate the formation of the persulphate of iron.